The most distinctive difference between the synchronous and asynchronous AC motor is whether their rotor speed is consistent with stator speed in magnetic field. If it is, then it is called synchronous AC motor, otherwise asynchronous AC motor.
When the number of the pole is fixed, there is a strict relationship between the motor speed and frequency, and this is called, with electrical engineering terms, synchronization. Asynchronous motor is also called induction motor, mainly used as a motor and its rotor speed is always less than synchronous motor at work.
The so-called "synchronization" means that the armature (stator) will form a rotating magnetic field in air gap after winding through the current, while the direction and the speed of the rotation is the same with that of rotor, so called synchronization. when it refers to induction motor, the speed of rotating magnetic field and rotor is relative, namely to produce torque.
Why does it synchronize and why does it synchronize?
The stator winding of synchronous motor and induction motor is the same. The main difference between them is the rotor structure. Dc field winding resides in synchronous motor rotor. Thus it requires additional excitation source through slip ring to introduce electricity current. Asynchronous motor rotor winding is short circuit, which produce current by electromagnetic induction. In contrast, the synchronous motor is more complex and higher in cost.
Synchronous and asynchronous motor are both belong to AC motor, which rely on 50 Hz ac power supply to make itself works. Asynchronous motor produces a rotating magnetic field when stator gets into alternating current (ac), while the rotor, when inducted, and generates magnetic fields. The interaction of two magnetic fields makes the rotor turns with the stator's rotating magnetic field. The rotor rotates slower than the stator rotating magnetic field, so there comes a torque. Thus it is called asynchronous machine. However, Synchronous motor stator and asynchronous motor is the same, but its rotor is deliberately added direct current to form constant magnetic field, so that the rotor can synchronize with the stator rotating magnetic field. So it is called synchronous motor.
To put it in a simple way, asynchronous motor rotor is not added dc excitation current while the synchronous motor rotor is added a direct excitation current in order to make the speed of the rotor identical with the rotational speed of magnetic field stator resulting from the torque between stator and rotor.
Why does synchronous motor rotor pass into the DC excitation current, rather than pass into the AC excitation current?
Assuming that the power frequency is 50HZ, the rotor can induct 50HZ in the stator windings when it gets through into DC excitation current. After the rotor get through the AC excitation current, it can disintegrate into positive and negative rotating magnetic field. The speed of positive rotational magnetic field adding the rotor rotational speed can induce 100HZ in the stator windings; the rotational speed of negative rotational magnetic field offset rotor rotational speed, relatively static to the stator winding, and producing no electric potential.